This invention relates to a three dimensional coordinate measuring apparatus which is for example to determine the location of a point on a three dimensional thing or a contour of the same.
There are two kinds of three dimensional coordinate measuring apparatuses. One kind includes an orthogonal coordinate mechanism having a probe and three movable elements, the first element being movable in a direction of the X coordinate axis of an orthogonal coordinate system defined in the measuring apparatus, the second element being movable in the direction of the Y coordinate axis, the third element being moveable in the direction of the Z coordinate axis so that the probe can be moved in any direction in the three dimensional space by the combination of the movements of the above three movable elements to detect the location of a point on a three dimensional thing or to trace a contour of the same. In this kind of apparatus, scales are provided along the coordinate axes, so that the coordinate components of points on a thing detected or traced by the probe are directly measured by the respective scales. Most of the three dimensional coordinate measuring devices are of this kind.
The other kind of measuring apparatus includes a polar coordinate mechanism. FIG. 1 shows the construction principle of the polar coordinate mechanism. As shown, the mechanism includes rigid rods R.sub.1, R.sub.2, R.sub.3, R.sub.4, connecting members P.sub.1, P.sub.2, P.sub.3, Q connecting the rods articulatedly and angle measuring means for measuring angles between the adjacent rods so that the distance and direction of the tip P.sub.0 of the rod R.sub.4, which acts as a probe, with respect to the fixed end of the rod R.sub.1 is determined based on the lengths l.sub.1, l.sub.2, l.sub.3, l.sub.4 and the angles .alpha., .beta., .delta., .theta.. The measuring apparatus includes a processing unit and the coordinate components of the location of the tip of the rod R.sub.4 in a predetermined orthogonal coordinate system are determined based on the distance and direction of the tip P.sub.0 by the processing unit.
The most advantageous feature of the measuring apparatus with the polar coordinate mechanism is that the polar coordinate mechanism includes a plurality of articulations so that the tip of the rod R.sub.4 can approach a desired point on a thing to be measured from any direction, whereby the apparatus can easily effect the measurement, even one which is difficult or impossible for the measuring apparatus with the orthogonal coordinate mechanism.
On the contrary, in measuring the contour of a thing defined by a section parallel to a plane including two coordinate axes of the coordinate system defined in the apparatus, for example, the X-Z plane with the measuring apparatus having the orthogonal coordinate mechanism the probe can be made to be movable only in a plane including the section by fixing the element movable in the Y coordinate axis direction of the orthogonal coordinate mechanism at the location of the section in the Y coordinate axis direction, in the measuring apparatus having the polar coordinate mechanism, it is impossible to restrain the movement of the probe in the Y coordinate axis direction as in the above mentioned apparatus, so that it is quite difficult for the operator to manually move the probe following the contour with accuracy and thus the measured coordinates with respect to the contour contain errors which exceed an allowable value. Further, when a contour of a thing defined by a section which is not parallel to a plane including two coordinate axes of the coordinate system is measured, the same problem occurs in the apparatus with the orthogonal coordinate mechanism as well as the other type apparatus. In the prior art, to dissolve such problem, marking-off along the contour or attaching a narrow and elongated tape on the contour is effected to guide a probe prior to the measurement and the operator must always observe the coordinate components concerning the contour generated by the apparatus during the measurement. Accordingly, it requires quite much time and labor.